The molybdenum-containing nitrogenase contains an iron-molybdenum cofactor, whose synthesis involves at least six nif genes. Genes corresponding to nifE, N, B, and V occur in proximity in Clostridium pasteurianum, with nifN-B occurring as one gene and with nifVω and nifVα in place of nifV. Between nifN-B and nifVωVα, we found a gene whose sequence is similar to chlJ of Escherichia coli. chlJ is part of the chlD locus, which is involved in Mo transport. C. pasteurianum actively accumulates Mo in a process coregulated with nitrogen fixation. We propose that nifC is involved in Mo transport. The expression of nifC may be coregulated with nitrogen fixation because of the presence of nif-distinctive promoter and upstream sequences preceding nifC-nifVω-nifVα. NifC contains a region typical of integral membrane proteins. Our findings suggest the involvement of a membrane-located nif gene product in Mo transport in C. pasteurianum. © 1990.